A disc brake is a device which operates to slow down or stop the movement of a vehicle by pressing the friction pads which are actuated by a hydraulic piston onto the side surfaces of a steel disc which rotates concomitantly with a wheel.
Pistons of a conventional disc brake have been made of a steel which has a high heat conductivity. Accordingly, when a vehicle is frequently being stopped or slowed down, the heat produced by frictional contacts between the friction pads and the steel disc is transmitted from the friction pads to a brake fluid via the steel piston to raise the temperature of the brake fluid. The high temperatures of the brake fluid may become a cause for a vapor lock which will give rise to undesirable conditions.
When the brakes are applied, high pressures are also exerted onto the pistons of a disc brake. If there were flaws or rusts in or on the sliding surfaces of the pistons, they may cause the brake fluid to leak. In order to prevent the development of flaws and rust, pistons equipped on conventional disc brakes have been coated with a hard chromium plating. The plating is expensive and disadvantageous in raising the manufacturing cost of a vehicle.